The present invention relates to scaling of image signals. It is advantageously used for up- or downscaling video frames, but also applicable to still images.
Downscaling of digital images, i.e. representing a high-resolution image signal at a lower resolution, requires an anti-aliasing operation to avoid viewing artifacts in the downscaled image.
Downscaling is used in TV sets for converting high-definition (HD) contents, or pictures to do picture overlays. Typical downscaling ratios can be 4 or more. Downscaling is also used in set top boxes for similar reasons. In these applications, upscaling can also be useful for displaying images received at a resolution lower than that available for display. Downscaling is also used in digital cameras, where multi-megapixel images shot by the camera module must be rendered on the small display of the camera which typically has between 0.2 and 1 megapixel only.
In order to perform a proper anti-aliasing, the input image is filtered with filters whose size should increase in proportion to the downscaling ratio. This usually requires an amount of logic which increases with the downscaling ratio. In the case of vertical downscaling, more line buffers are also required, which increases the size and the cost of the component.
There are various ways of performing downscaling with anti-aliasing:                using an expensive anti-aliasing filter having a large number of multipliers and, for vertical scaling, of line buffers;        using a non optimal anti-aliasing filter to reduce the logic and memory cost, but with degraded visual quality;        using multiscale anti-aliasing filters to reduce the implementation cost of the anti-aliasing filtering, but at the expense of more complex architectures.        
WO 2005/106787 discloses an apparatus for downscaling digital matrix images by selected ratios. A weighted sum of the values of the input pixels of the matrix image in the area of each output pixel is calculated with weighting coefficients given by the dimension shares of the input pixels in the area of the output pixel, corrected by a scaling factor. This technique can facilitate the implementation of the downscaling operation with a filter of variable size. But the filter that it uses has poor spectral properties, leading to sub-optimal anti-aliasing and degradation of image quality.
There is thus a need for another scaling method providing a good compromise between simplicity of implementation and quality of the downscaled anti-aliased images.